Centrifugal Pump Theory

ENERGY TRANSFER

Hydraulics or fluid dynamics has the primary influence on the geometry of a roto-dynamic pump stage—of all the engineering disciplines involved in the design of the machine. It is basic to the energy transfer or pumping process. Staging is also influenced by the other disciplines, especially in high-energy pumps. The basic energy transfer relationships need to be thoroughly understood to achieve a credible design and to understand the operation of these machines. Action of the mechanical input shaft power to effect an increase in the of energy of the pump-age is governed by the first law of thermodynamics. Realization of that energy in terms of pump pressure rise or head involves losses and the second law of thermodynamics.

The First Law of Thermodynamics

Fluid flow, whether liquid or gas, through a centrifugal pump is essentially adiabatic, heat transfer being negligible in comparison to the other forms of energy involved in the energy transfer process. (Yet, even if the process were not adiabatic, the density of a liquid is only weakly dependent on temperature.) Further, while the delivery of energy to fluid by rotating blades is inherently unsteady (varying pressure from blade to blade as viewed in an absolute reference frame), the flow across the boundaries of a control volume surrounding the pump is essentially steady, and the first law of thermodynamics for the pump can be expressed in the form of the adiabatic steady-flow energy equation as follows :

Here, shaft power Ps is transformed into fluid power, which is the mass flow rate times the change in the total enthalpy (which includes static enthalpy, velocity energy per unit mass, and potential energy due to elevation in a gravitational field that produces acceleration at rate g) from inlet to outlet of the control volume (Figure 1). When dealing with essentially incompressible liquids, the shaft power is commonly expressed in terms of “head” and mass flow rate, as in Equation:

Energy transfer in a centrifugal pump

The change in H is called the “head” _H of the pump; and, because H includes the velocity head V2/2g and the elevation head Ze at the point of interest, ΔH is often called the “total dynamic head.” _H is often abbreviated to simply “H” and is the increase in height of a column of liquid that the pump would create if the static pressure head p/ρg and the velocity head V2/2g were converted without loss into elevation head Ze at their respective locations at the inlet to and outlet from the control volume; that is, both upstream and downstream of the pump.

SELECTION OF PUMPS

Given the variety of pumps that is evident from the foregoing system of classification, it is conceivable that an inexperienced person might well become somewhat bewildered in trying to determine the particular type to use in meeting most effectively the requirements for a given installation. Selecting and Purchasing Pumps guide that provides the reader with reasonable familiarity regarding the details that must be established by or on behalf of the user in order to assure an adequate match between system and pump.

None of these, however, provide a concise comparison between the various types, and Figure 1


FIGURE 1 Approximate upper limit of
pressure and capacity by pump class

has been included here to do just that, at least for the basic criteria of pressure and capacity. The lines plotted in Figure 1 for each of the three pump classes represent the upper limits of pressure and capacity currently available commercially throughout the world. At or close to the limits shown, only a few sources may be available, and pumps may well be specially engineered to meet performance requirements. At lower values of pressure and capacity, well within the envelopes of coverage, pumps may be available from dozens of sources as pre-engineered, or standard, products. Note also that reciprocating pumps run off the pressure scale, whereas centrifugals run off the capacity scale. For the former, some highly specialized units are obtainable at least up to 150,000 lb/in2 (10,350 bar) and perhaps slightly higher. For the latter, custom-engineered pumps would probably be available up to about 3,000,000 U.S. gal/min (680,000 m3/h), at least for pressures below 10 lb/in2 (0.69 bar).

CLASSIFICATION OF PUMPS

Classification of pump bases on the fundamental of the applications which they will serve their own functions, properties of constructed material, the type of the liquids that they handle, and even their direction of installed formation. All kind of these classifications are limited by the subjects of their unique characteristics, So it will not overlap each others substantially.

The essential classification of basic system is defined by whichever energy added into the fluid of pumps. It causes the principle of identification can be shown and implemented such as delineates specific geometries.
FIGURE 1 Classification of dynamic pumps

Therefore, this classified system can be related directly to the pump itself so any external of consideration can be done such as material or construction consideration. Due to this system, pumps can be divided of two categories :
1. Dynamic flow rate
Energy is continuously added into the fluid to increase its own velocities so it will be useful for the machine, including of reduction velocity in discharge section.

2. Displacement

In which, energy forms, like application of force is periodically added into moveable fluid boundaries, so will result the increasing of fluid pressure as a value of fluid movement through out the discharge line such as ports of valves.

FIGURE 2 Classification of displacement pump

For further classifications, dynamic pump can be classified into various centrifugal pump and others. Meanwhile, the displacement pump can be divided into rotary and reciprocating pump. As a summary of pumps classification and sub division of dynamic pumps, presented by Figure 1, outline form significant classification. About various specific types of commercial displacement pumps are indicated in illustration of Figure 2.